The term membrane switches is used herein to refer to electrical switches constructed of at least two layers of plastic film spaced from one another so that a surface of one film faces a surface of the other film. The two facing surfaces each carry a conductive pattern that is typically printed on the film surfaces with conductive inks or applied onto the films by vacuum metalizing techniques. The conductive patterns include contact zones connected by conductive tracks that connect the conductive patterns to external circuitry. The films are spaced from each other by a spacer layer between the two facing surfaces of the films, which is generally a patterned adhesive layer or a die-cut plastic film having apertures positioned between contact zones of the conductive pattern on the surface of one film and contact zones of the conductive pattern on the facing surface of the other film. Thus, in the deactivated condition, the spacer layer maintains the contact zones of one circuit pattern spaced apart from the other conductive pattern. One of the films is arranged to be flexed or depressed toward the other in the activated condition of the switch to thereby establish an electrical circuit between a selected contact zone of one pattern with its mating contact zone of the conductive pattern on the other layer, thereby generating a signal that can be detected by external circuitry to which the membrane switch is connected.
Membrane switches presently find widespread use in installations in which a sealed or protected switch or operating panel is desirable. For example, they are employed in equipment which requires manual data entry such as computer keyboards, terminals, cash registers and the like. Also, membrane switches are widely used as a control or instrument panel for appliances such as washers and microwave ovens, industrial controls, copy machines, and the like, in which finger touch micro-motion actuation is a useful feature.
Membrane switches are actuated by an operator depressing, with a finger, a designated key area of one layer of the switch towards the other layer, the two layers being closely spaced from one another so that only a slight amount of movement of the layer which is depressed is required for proper operation.
Membrane switches have been primarily of the resistance type in which two conductive zones or contacts are closed and electric current flows through them in proportion to the voltage applied across them. Resistance switches are well known in the art and particular constructions for this type of membrane switch are disclosed in a number of patents assigned to the assignee of this application, including e.g., U.S. Pat. Nos. 4,217,473, 4,218,600 and 4,264,797. A more recent development in the art of membrane switches is the capacitance type of membrane switch which operates upon the change in capacitance between spaced conductive zones and are described for example, in U.S. Pat. No. 4,359,720 assigned to the assignee of this application.
Membrane switches are presently made either as a laminated construction or a folded construction. In a laminated construction, two plastic film layers that are to carry conductive patterns are made from separate or independent panels cut to the desired configuration. A conductive pattern is applied to a surface of each panel, the two panels are then properly registered relative to one another and laminated together to form a completed switch with a spacer layer positioned between the two panels. The spacer layer may be an adhesive layer or a plastic film carrying adhesive on both of its surfaces, and thus serve to laminate the two ciruit-carrying panels together, or a plastic film without adhesive. Most commercial membrane switches are made in this fashion at the present time. In a folded construction, by comparison, a single sheet of plastic film is die-cut to form the two panels that are to carry conductive patterns in a configuration in which the panels are joined together along a fold line. A conductive pattern including contact zones is applied to a surface of one panel and another conductive pattern with contact zones is applied to what will be the facing surface of the other panel with the sheet in a flat condition. Thereafter, one contact-carrying panel is folded along the fold line so as to overlie the other contact-carrying panel, after which the two panels are joined together with a spacer layer of adhesive or die-cut film having adhesive on its opposed surfaces.
My present invention relates to the folded type of membrane switch.
Folded membrane switches are described in a number of U.S. patents, including the following of which I am aware:
______________________________________ 3,745,288 4,207,444 3,909,564 4,264,477 3,911,234 4,336,529 3,982,081 4,356,358 4,028,509 4,365,408 4,066,851 4,440,990 4,145,584 4,440,999 4,451,714 ______________________________________
The prior art folded style of membrane switches such as described in the above patents have a common structural feature: the panels that carry the contact zones are contiguous with one another along the fold line about which one of the panels is to be folded over the other. The panels are arranged in either a side-by-side or end-to-end juxtaposition along the fold line. Thus, in these prior art constructions, the sheet of film is die cut into a blank of the desired size and configuration in which one contact-carrying panel is connected to the other contact-carrying panel along the fold line about which one panel is folded relative to the other.
The prior art folded membrane switches such as described above have a number of problems. (1) The fold line is at an edge of the switch between the two panels carrying contact zones. This produces a bulge along the edge of the membrane switch, that can be visible through the faceplate or other covering panel which generally comprises a printed plastic film carrying graphic information for identification of the individual key sites. The bulge produced by the fold line is objectionable, and it would be desirable to eliminate it in a folded membrane switch construction. (2) In order to reduce the objectionable characteristic of the fold line as discussed above, the fold at the edge of the switch is often made as tight as possible to minimize the bulge. This exerts high stress on the conductive pattern at the fold line, which can result in switch failure. (3) The portion of the conductive pattern crossing the fold line is left uncovered in the typical folded membrane switch construction. This uncovered portion is thereby exposed to external contaminants which can attack and degrade the circuit pattern. (4) In addition, membrane switches are often applied to an apparatus by means of a layer of pressure sensitive adhesive along the bottommost surface of the switch. However, since it is undesirable that the application adhesive extend around the fold line to the top of the switch, the adhesive is generally undersized along the bottom surface of the switch so that there is a portion of the bottom surface adjacent the fold line which is not covered by application adhesive. This means there is no adhesive attachment of the switch to a panel or other article to which the switch is to be attached along this area adjacent the fold line. (5) While the two panels of a folded switch are joined to each other by a spacer layer, the effect of having the fold line along the edge of the two panels is to cause the panels to separate over a period of time. This is undesirable and can lead to malfunctioning of the switch.